The present invention relates to a circuit and method for providing a signal strength sensitive current and, more particularly, to a circuit and method for producing a meter drive signal or the like in an intermediate frequency (IF) limiter stage, the magnitude of which varies linearly with the logarithm of an input signal supplied to the IF limiter stage.
Prior art receivers such as FM receivers having multi-stage IF limiter stages are well known in the art. Further, there are prior art receivers comprising such IF limiter systems that also include meter drive circuits which produce a current output signal that, among other things, may be utilized to drive a signal strength indicating meter. The current output signal may also be used as an aid in production alignment of such receivers as well as providing other functions such as squelch control.
One prior art meter drive circuit which has been incorporated into FM receivers derives the current drive signal from the IF limiter system of the receiver by capacitively coupling separate rectifier circuits to one or more of the several amplifier stages of the IF limiter respectively. The amplified signals from the individual amplifier stages of the IF limiter are measured by the individual rectifier circuits which peak rectify the signal appearing at the output of the particular amplifier stage by essentially clamping the peak carrier frequency amplitude at a predetermined signal level. Hence, as the carrier frequency amplitude increases, the average DC value of the carrier frequency also increases. The clamped signal is then applied to the base of a transistor which provides an output current drive signal that includes both the DC and carrier frequency components. In order to provide a useful signal, the carrier frequency component of the aforementioned output signal must be filtered. Because of the relatively large carrier frequency component, strict filter requirements are required which is undesirable due to, among other reasons, the cost of the required filter components.
Hence, there is a need for a much simpler and cheaper circuit and method for deriving a signal that is sensitive to the strength of an input signal supplied to a receiver and which requires less filtering than prior art meter drive circuits.